Normally, in an electromagnetic pump, an electromagnetic plunger and a pressure piston connected to the electromagnetic plunger are made to reciprocate by means of an interrupted electromagnetic force of an electromagnetic coil and the pumping action of the pump is effected by a suction valve and a discharge valve working together. In order to keep the discharge pressure of the electromagnetic pump constant, so far, there have been two systems developed.
One system which has been developed is a relief valve system for returning the extra outflow to the suction side of the pump, and the other system which has been developed is a pressure sensitive control decompression valve system that senses the pressure at the discharge side of the pump to control the outflow by throttling down the path cross-sectional area at the discharge side of the pump.
The relief valve system is described in the specification of U.S. Pat. No. 3,877,841, wherein a predetermined discharge pressure is set by returning the fluid to the suction side of the pump from the discharge side of the pump by means of a relief valve; however, such a pump always has a large constant piston stroke as the performance must be set above the required outflow to compensate for the performance drop due to the temperature rise of the electromagnetic coil and to provide compensation for changes of the discharge pressure in response to a fluctuation (primarily, .+-.10%) of the input voltage. The required large constant piston stroke gives rise to unnecessary excess vibration or noise.
Also, since the piston stroke is large, there is a need to provide a pressure accumulator for the prevention of pressure pulsation. Furthermore, in many cases, the relief valve is made of a soft material such as rubber in order to produce the desired performance as far as the pump structure is concerned; the use of such material results in accelerated pump deterioration and severely limited pump service life. The failure of the relief valve reduces the discharge pressure and in the case where the pump is used in conjunction with a combustor, it results in incomplete combustion of the pumped fuel.
The pressure sensitive control decompression valve system is arranged such that a decompression valve is provided in the flow path at the discharge side of the pump and the decompression valve is displaced by the discharge side pressure to control the outflow of the discharge side, whereby the piston stroke can be set to a required minimum in proportion to the discharge outflow; the smaller piston stroke results in a lower noise level than that for the relief valve system and furthermore, no accumulator is required, both of which are significant advantages. Existing pressure sensitive control valve systems can be roughly classified into the following two methods with respect to their pressure receiving mechanisms configuration.
One method uses a diaphragm as the pressure receiving mechanism and has drawbacks which occur frequently, such as the loss of effect of the diaphragm because of the fact that the fluid used saturates the diaphragm material due to long hours of use, and because of the fact that fluid is collected in the chamber on the housing side of the spring, thus varying the pressure on the diaphragm or causing the breakage of the diaphragm.
A second method uses a metal bellows as the pressure receiving mechanism and has other drawbacks, such as cracks therein due to long hours of use in an electromagnetic pump having large pressure fluctuations or lack of practicality with regard to high bellows fabrication costs. Furthermore, as described in the foregoing, the diaphragm and the bellows, which form the pressure receiving mechanisms, are broken due to the high pressure and the change of chemical/physical property thereof caused by the fluid used and deterioration due to the influence of physical fatigue caused by the pressure pulsation; there is also the danger of causing excessive outflow at the maximum performance of the electromagnetic pump if the pumped fluid is applied at an excessively high pressure directly to a nozzle. As described in the foregoing, many various problems occur in the conventional pressure adjustments of the discharge pressure.